October 1, 1898.] 



KNOWLEDGE 



217 



Founded in i88i by RICHARD A. PROCTOR. 



LONDON : OCTOBER 1, 1898. 



CONTENTS. 



An Esker in the Plain. By Gbestiile A. J. Cole, m.r.i.a.. 



F.G.s. {Illustrated) ... 

 The Sea-Squirt By E. SxExnorsE, A.it.c s., b.so. ... 



The Affinities of Flowers.— The Bladderwort and its 

 Relatives. By Feli.x Oswatd, b.a., ii.sc. (Illiislrateil) 



Ethnology at the British Museum. By K. Ltdekkeb. 

 {Illustrated) ... 



The Fourth International Congress oT Zoology 



The Great Sunspot and the Aurora. By E. Waltee 



MaUNDEB, F.B.A.S. (Illli.<tiraterl nnd PlafA 



Letter:— J. M-R 



Science Notes 



Notices of Books 



Shoet Notices 



Books Received 



British Ornithological Notes. Conducted by Haebt F. 



WiTHEBBT, E.Z.g., M.B.O.IT 



Sunspots and Life. By Alex. B. MacDowall, m.a. 



{illustrated) " 



Economic Botany. By John R. Jackson, a.i.s., etc. ... 

 Notes on Comets and Meteors. By W. F. DKNNiKa, 



F.E.A.3 



The Face of the Sky for October. By A. Fowxeb, 



F.B.A.S. 



Chess Column. By C. T>. Locock, b.a 



217 

 220 



223 

 226 



228 

 229 

 2.'!0 

 230 

 233 

 233 



234 

 235 



238 

 239 



AN ESKER IN THE PLAIN. 



By Grenville A. J. Cole, m.r.i.a., fg.s., Professor of 

 Genlogy in tlw Eoyiil Collei/e of Science for Ireland. 



THE gravel ridges of the Irish plain have been 

 already mentioned* as a welcome feature in its 

 landscapes. These " green hills," with their 

 pleasant grassy slopes, have often given a name 

 to groups of houses clustered near them ; and 

 here and there they, were seized on long ago as sites 

 for commanding forts. The Irish word eiscir means " a 

 ridge," and there is a hamlet called " Esker " to this day 

 on a gravel bank near Lucan. The term has, however, 

 become a scientific one, through the interest roused among 

 geologists by the characters of many of these ridges ; and 

 General Portlock.t Mr. G. H. Kinahan, and, finally, Mr. 

 Maxwell Close, J have distinguished between eskers proper 



* Knowledqe, Yol. XXI., p. 75. (April. 1898.) 

 t "Report on Geology of Londonderry, Tyrone, etc.," 1843, p. 639. 

 X " General Glaciation of Ireland," Jour. H. Geol. Soc, Ireland 

 Vol.1. (1867), p. 211 and p. 212, footnote. 



and the fairly parallel banks of drift, or dnimUns, which 

 are found so abundantly in glaciated countries. 



We need not go far from Dublin to find a typical little 

 esker. Four miles south-west of the city, out in the lime- 

 stone plain, the main road to Tallaght makes a sudden 

 rise, and reaches the crest of a green ridge on which the 

 hamlet of Balrothery stands (Fig. 1). Gravel pits have been 

 opened on either hand, and a by-road turns off along the 

 ridge, which it follows for some three miles to Crumlin. 

 Such a road is in itself a feature of an esker ; these dry 

 raised causeways offered themselves to the ancients ready- 

 made ; and the fact that they seldom ran in a straight 

 line was not in those days of much importance. If we 

 start from Balrothery, we at once note that the esker is 

 formed of irregular beds of pebbles, with occasional yellow 

 sands. At the summit it is little wider than the road, and 

 falls with a slope of twenty degrees on either hand ; from 

 its base there is a gentler slope to the ordinary level of 

 the fields, doubtless due to the washing down of detritus 

 from the ridge. Before us, planted on the crest, rises the 

 tower of Tymon Castle, one of the defences of Norman 

 Dublin against the Irish ; and the road has to give way 

 and descend round about it. The west slope has here an 

 angle of nearly thirty degrees (Fig, 2). Soon we reach Green 

 Hills, where the inhabitants are engaged in quarrying, 

 and where large sections have been opened in the esker. 

 Here the ridge broadens and becomes less defined, and 

 finally breaks up into a number of mere mounds of gravel. 



When once recognised, such a feature will be picked out 

 again and again in a traverse of the Irish plain. Gravels 

 are common on its surface, largely composed of limestone 

 pebbles, with a sprinkling of other rocks, which can gene- 

 rally be traced to the highlands of the country along one 

 or other line of ice-drift. The pebbles of the plain are 

 ground and striated on their surfaces, and clearly were 

 at one time under solid ice, or embedded in its moving 

 layers. When, however, we examine the material of the 

 eskers, we find the same pebbles, but with subsequent 

 signs of water-action. Here and there the old strife 

 remain ; but in most cases further rounding and abrasion 

 have gone on. The bedding, whether in the rough layers 

 of the gravel, which are seen to dovetail into one another 

 in the sections, or in the delicate stratification of the brown 

 and yellow sands, reminds us at once of the river-deposits 

 that are laid bare by Alpine streams. But in the esker 

 the form of a stream-deposit is reversed ; instead of an 

 alluvial mass, filling up the groove of a valley-floor, and 

 widening from below upwards, we have the narrower part 

 at the top, and a pebbly ridge has been heaped up without 

 visible retaining walls. 



The sharp ridges formed by the lateral moraines, as a 

 glacier shrinks in its own bed, will come to the mind of 

 any traveller. But these occur in pairs, or series of pairs, 

 marking successive halting-points in the transverse shrink- 

 age of the glacier. They curve round, moreover, towards 

 the terminal moraine at the nose of the glacier, and are 

 altogether more systematically disposed than these eskers 

 of the Irish plain. Further, their materials are just 

 dropped off the edges of the ice, and are not specially 

 waterworn. 



Elvers, again, do not form isolated ridges of detritus, 

 although they may raise their courses above a plain on 

 broad strips of pebbly land, which they themselves have 

 formed. For a long time, the movements of currents in a 

 shallow sea was invoked to account for the building of 

 eskers, and their various curvings and bays were held to 

 mark swirls of water along which the pebbles had become 

 accumulated. Marine shells, however, could not be found 

 in the esker-gravels, though they are plentiful in some 



